Section 1.0: Types of Nuclear Weapons http://astro.uchicago.edu:80/home/web/jeffb/abomb/nfaq1.html VERSION 1.11 July 14, 1995 COPYRIGHT CAREY SUBLETTE Free use of this material is hereby granted provided that proper attribution is given. Nuclear weapons can be grouped into different classes based on the nuclear reactions that provide their destructive energy, and on the details of their design. The popular division of nuclear weapons into fission bombs and fusion bombs is not entirely satisfactory. The spectrum of weapon design is more complex than this simple classification implies. All nuclear weapons so far invented require fission to initiate the explosive release of energy. Weapons that incorporate fusion fuel can do so in various ways, with different intended effects. This section attempts to survey the basic types of bomb designs systematically. More detailed discussions of the physics and design principles of each type will be covered in more detail in later sections. Ľ1.1 Terminology Ľ1.2 U.S. Nuclear Test Names Ľ1.3 Units of Measurement Ľ1.4 Pure Fission Weapons Ľ1.5 Combined Fission/Fusion Weapons Ľ1.6 Cobalt Bombs Section 1.1: Terminology http://astro.uchicago.edu:80/home/web/jeffb/abomb/nfaq1_1.html A variety of names are used for weapons that release energy through nuclear reactions - atomic bombs (A-Bombs), hydrogen bombs (H-Bombs), nuclear weapons, fission bombs, fusion bombs, thermonuclear weapons (not to mention "physics package" and "device"). A few comments about terminology is probably in order. The earliest name for such a weapon appears to be "atomic bomb". This has been criticized as a misnomer since all chemical explosives generate energy from reactions between atoms - that is, between intact atoms consisting of both the atomic nucleus and electron shells. Further the fission weapon to which "atomic bomb" is applied is no more "atomic" than fusion weapons are. However the name is firmly attached to the pure fission weapon, and well accepted by historians, the public, and by the scientists who created the first nuclear weapons.Since the distinguishing feature of both fission and fusion weapons is that they release energy from transformations of the atomic nucleus, the best general term for all types of these explosive devices is "nuclear weapon" (hence the name of this FAQ). Fusion weapons are called "hydrogen bombs" (H-Bombs) because isotopes of hydrogen are principal components of the nuclear reactions involved. In fact, in the earliest fusion bomb designs deuterium (hydrogen-2) was the sole fusion fuel. Fusion weapons are called "thermonuclear weapons" because high temperatures are required for the fusion reactions to occur. Section 1.2: U.S. Nuclear Test Names http://astro.uchicago.edu:80/home/web/jeffb/abomb/nfaq1_2.html Before discussing U.S. nuclear tests, the designation system used to identify the tests and each bomb that is tested should be clarified. Each test bomb has a code name that identifies it, the actual test has another code name. Thus the first atomic bomb was called Gadget, and it was tested in operation Trinity. The early test operations were conducted as part of a test series, a large scale operation where many scientists, support technicians, military personnel, etc. assemble in order to set off and observe a number of devices over several weeks or months. This test series has another code name. For example the second and third test explosions of nuclear weapons (which were the fourth and fifth nuclear explosions of course) were part of the Crossroads test series. The two tests were designated Able and Baker. In the early test series, the same test names were reused several times. Thus there was an Able test in the Crossroads, Ranger, Buster-Jangle, and Tumbler-Snapper test series. To unambiguously identify each test the convention is to list the series code name, followed by the test name: Crossroads Able, Ranger Able, and so on. After mid-1952 unique test names began to be used, so that this convention was no longer strictly necessary. However it is useful to specify the series as well, so this FAQ makes the practice of identifying all tests by the series-test combination. After 1961 the test series system was dropped as underground testing in Nevada became routine, all of which are considered part of the Nevada Series. There was a final series of open air testing in the Pacific (the Pacific Series)in 1962, and a few special test programs (Plowshare, Vela Uniform Seismic Detonation) but starting in 1961 all tests are identified simply by their test names. The code names of the actual devices are generally not well known. Many remained classified until recently (or still are). Since a bomb can only be tested once, identifying the device by the test in which it was detonated is unambiguous. In the open literature the test name has usually been used to designate the bomb that was tested, a convention followed here as well. Section 1.3: Units of Measurement http://astro.uchicago.edu:80/home/web/jeffb/abomb/nfaq1_3.html The yield of nuclear weapons is usually measured in megatons, kilotons (or even just tons), depending on yield. These units of measurement are derived from attempts to compare the explosive force of a bomb to conventional explosives, the original intention was to equate it with tons of trinitrotoluene (TNT) - a workhorse military explosive. This presented problems very quickly. Which tons are being referred to short tons, long tons, metric tons (tonnes)? And the explosive force of TNT is not exactly a universal constant. The energy release is affected by such things as charge density, degree of confinement, and temperature. Energy outputs ranging over 980-1100 calories/g are reported. To simplify things, kilotons (megatons, etc.) were redefined to be a metric unit equal to exactly 10^12 calories (4.186x10^12 joules). Thus treating kilotons as a metric mass measurement (kilotonnes) of TNT gives a value of 1000 c/g, well within the reported range, while treating it as "kilo short tons of TNT" gives 1102 c/g, at the extreme upper end of the reported range. Thus a kiloton can be called a "kilo metric ton of TNT" and a "kilo short ton of TNT" with about equal validity. Section 1.4: Pure Fission Weapons http://astro.uchicago.edu:80/home/web/jeffb/abomb/nfaq1_4.html These are weapons that only use fission reactions as a source of energy.Fission bombs operate by rapidly assembling a subcritical configuration of fissionable material into one that is highly supercritical. The original atomic bombs tested in July, 1945 and dropped on Japan in August, 1945 were pure fission weapons. There are practical limits to the size of pure fission bombs. Larger bombs require more fissionable material, which 1) becomes increasingly difficult to maintain as a subcritical mass before detonation and 2) makes it harder to assemble into a high efficiency supercritical mass before stray neutrons cause predetonation. Due to secrecy, and the boosting issue described below, it is somewhat difficult to identify the largest pure fission bomb ever tested for certain. It may have been the 500 kt Ivy King test (11/15/52), but the UK 720 kt test (5/31/57) is also a candidate. Section 1.5: Combined Fission/Fusion Weapons http://astro.uchicago.edu:80/home/web/jeffb/abomb/nfaq1_5.html All nuclear weapons that are not pure fission weapons use fusion reactions to enhance their destructive effects. All weapons that use fusion require a fission bomb to provide the energy to initiate the fusion reactions. This does not necessarily mean that fusion generates a significant amount of the explosive energy, or that explosive force is even the desired effect. Section 1.5.1: Boosted Fission Weapons http://astro.uchicago.edu:80/home/web/jeffb/abomb/nfaq1_5.html The earliest application of fusion to useful weapons was the development of boosted fission weapons. In these weapons several grams of a deuterium/tritium gas mixture are included in the center of the fissionable core. When the bomb core undergoes enough fission, it becomes hot enough to ignite the D-T fusion reaction which proceeds swiftly. This reaction produces an intense burst of high-energy neutrons that causes a correspondingly intense burst of fissions in the core. This greatly accelerates the fission rate in the core, thus allowing a much higher percentage of the material in the core to fission before it blows apart. Typically no more than about 20% of the material in a pure fission bomb will split before the reaction ends (it can be much lower, the Hiroshima bomb was 1.4% efficient). By accelerating the fission process a boosted fission bomb can raise this to as much as 50% (an unboosted 20 kt bomb can thus become a 40 kt bomb). The actual amount of energy released by the fusion reaction is negligible, about 1% of the bomb's yield, making boosted bomb tests difficult to distinguish from pure fission tests (detecting traces of tritium is about the only way). Due to the marked increase in yield today most fission bombs are boosted, including those used as triggers in true fission-fusion weapons. Boosting multiplies the yield of fission bombs, but still has the same fundamental fission bomb design problems for high yield designs. The boosting technique is most valuable in small light weight bombs that would otherwise have low efficiency. Tritium is a very expensive material to make, and it decays at a rate of 5.5% per year, but the small amounts required for boosting make its use economical. Section 1.5.2: Staged Fission-Fusion and Fission-Fusion-Fission Weapons http://astro.uchicago.edu:80/home/web/jeffb/abomb/nfaq1_5.html These weapons use fusion reactions involving isotopes of light elements (e.g. hydrogen and lithium) to remove the yield limits of fission and boosted fission designs, to reduce weapon cost by reducing the amount of costly enriched uranium or plutonium required for a given yield, and to reduce the weight of the bomb. The fusion reactions occur in a package of fusion fuel that is physically separate from the fission trigger, thus creating a two-stage bomb (the fission trigger counts as the first stage). The energy produced by the fusion second stage can be used to ignite an even larger fusion third stage. Multiple staging allows in principle the creation of bombs of virtually unlimited size. The fusion reactions are used to boost the yield in two ways: 1) By directly releasing a large amount of energy in fusion reactions; 2) By using high-energy or "fast" neutrons generated by fusion to release energy through fissioning of U-238 in the form of natural or depleted uranium, which is ordinarily considered non-fissionable. Bombs that release a significant amount of energy directly by fusion, but do not use fusion neutrons to fission U-238, are called Fission-Fusion weapons. If they employ the additional step of fast-fissioning U-238 they are called Fission-Fusion-Fission weapons. Bombs that are billed as "clean" bombs (a relative term) obtain a large majority of their total yield from fusion. These are always fission-fusion bombs, the fusion-fraction of these designs as demonstrated in tests has been as high as 97%. Fission-fusion-fission bomb are dirty, but they have superior "bang for the buck" and "pow per pound". They generate a large amount of fission fallout since fission accounts for much of their yield. A fission-fusion-fission weapon can have a fission fraction well above 50% (Ivy Mike, 11/1/52, had a fission fraction of 77%).The staging concept allows the use as fuel pure deuterium, or varying mixtures of lithium 6 and 7 in the form of a compound with deuterium (lithium 6/7 deuteride). These natural stable isotopes are vastly cheaper than the artificially made and radioactive tritium. Three stage designs have been tested and deployed to produce very high yield weapons. The largest nuclear explosion ever set off (50 mt) was a Soviet three stage fission-fusion-fission design. The test omitted the final fission blanket however, so that it was actually a _150 mt_ design! Section 1.5.3: The Soviet Layer Cake Design http://astro.uchicago.edu:80/home/web/jeffb/abomb/nfaq1_5.html Andrei Sakharov and Vitalii Ginzburg jointly devised a synergistic fission-fusion scheme dubbed the "Layer Cake" that was developed into a deliverable weapon by the Soviet Union prior to their development of the staged designs described above. This design is something of a hybrid and could be considered either a type of boosted fission device, or a one-stage type of fission-fusion-fission bomb. The layer cake design is so named because it used concentric shells of U-235/Pu-239, a U-238 fission tamper, a layer of lithium-6 deuteride and tritium, and a U-238 fusion tamper. The fisson bomb in the center started a coupled fission-fusion-fission chain reaction. Slower fission neutrons generated tritium from the lithium, which then fused to produce very fast neutrons that in turm caused additional fissions in the fusion tamper. In effect the fusion fuel acted as a neutron accelerator allowing a fission chain reaction to occur with a large normally non-fissionable U-238 mass. The fusion fraction is fairly small, 15-20%, and cannot be increased beyond this point. This design is also limited to the same yield range as pure fission and boosted fission weapons. Although apparently not used in any weapons now in service, it remains a viable design that should probably be considered distinct from other classes of nuclear weapons. Section 1.5.4: Neutron Bombs http://astro.uchicago.edu:80/home/web/jeffb/abomb/nfaq1_5.html Neutron bombs are nuclear weapons where the burst of neutrons generated by a fusion reaction is intentionally not absorbed, but allowed to escape. The intense burst of high-energy neutrons is the principle destructive mechanism. High energy neutrons are not readily stopped by most materials. The U.S. has developed neutron bombs for use as anti-missile weapons (the approximately 20 kt warhead for the Sprint missile), and as an anti-personnel weapon against armored forces (low kiloton weapons developed both as artillery and missile warheads). More than one approach may exist for designing these weapons, the issue is discussed in Section 4. Neutron bombs may generate a large part of their energy from fusion, but not necessarily. The Lance missile warhead developed by the U.S. gained about 60% of its energy from fusion, the rest from the fission trigger. Tactical neutron bombs are primarily intended to kill soldiers who are protected by armor. Armored vehicles are very resistant to blast and heat produced by nuclear weapons, but steel armor can reduce neutron radiation by only a modest amount so the lethal range from neutrons greatly exceeds that of other weapon effects. The lethal range for tactical neutron bombs can exceed the lethal range for blast and heat even for unprotected troops. Armor can absorb neutrons and neutron energy, thus reducing the neutron radiation to which the tank crew is exposed, but this offset to some extent by the fact that armor can also react harmfully with neutrons. Alloy steels for example can develop induced radioactivity that remains dangerous for some time. When fast neutrons are slowed down, the energy lost can show up as x-rays. Some types of armor, like that of the M-1 tank, employ depleted uranium which can undergo fast fission, generating additional neutrons and becoming radioactive. Special neutron absorbing armor techniques have also been developed, such as armors containing boronated plastics and the use of vehicle fuel as a shield. Section 1.6: Cobalt Bombs http://astro.uchicago.edu:80/home/web/jeffb/abomb/nfaq1_6.html This design is reminiscent of fission-fusion-fission weapons, but a thick cobalt metal blanket is used to capture the fusion neutrons to maximize the fallout hazard from the weapon (this is also called "salting"). Instead of generating additional explosive force (and dangerous fission fallout) from fast fission of U-238, the cobalt is transmuted into Co-60 (natural cobalt consists entirely of Co-59). Cobalt 60 has a half-life of 5.26 years and produces energetic (and thus penetrating) gamma rays. The Co-60 fallout hazard is greater than the fission products from a U-238 blanket because 1) many fission-produced isotopes have half-lives that are very short, and thus decay before the fallout settles or can be protected against by short-term sheltering; 2) many fission-produced isotopes have very long half-lives and thus do not produce very intense radiation; or 3) the fission products are not radioactive at all. The half-life of Co-60 on the other hand is long enough to settle out before significant decay has occurred, and to make it impractical to wait out in shelters, yet is short enough that intense radiation is produced. The idea of the cobalt bomb originated with Leo Szilard who publicized it in Feb. 1950, not as a serious proposal for weapon, but to point out that it would soon be possible in principle to build a weapon that could kill everybody on earth (see Doomsday Device in Questions and Answers). To design such a theoretical weapon a radioactive isotope is needed that can be dispersed world wide before it decays. Such dispersal takes many months to a few years so the half-life of Co-60 is ideal. Initially gamma radiation fission products from an equivalent size fission-fusion-fission are much more intense than Co-60: 15,000 times more intense at 1 hour; 35 times more intense at 1 week; 5 times more intense at 1 month; and about equal at 6 months. Thereafter fission drops off rapidly so that Co-60 fallout is 8 times more intense than fission at 1 year and 150 times more intense at 5 years. The very long lived isotopes produced by fission would overtake the again Co-60 after about 75 years. Zinc has also been proposed for salting. The isotope Zn-64, which makes up 48.9% of natural zinc, would be converted to Zn-65 which is a gamma emitter with a 244 day half-life. The advantages of Zn-64 is that its faster decay leads to greater initial intensity. Disadvantages are that since it makes up only half of natural zinc, it must either be isotopically enriched or the yield will be cut in half; and that it is a weaker gamma emitter than Co-60, putting out only one-fourth as many gammas for the same molar quantity. Assuming pure Zn-64 is used, the radiation intensity of Zn-65 would initially be twice as much as Co-60. This would decline to being equal in 8 months, in 5 years Co-60 would be 110 times as intense.Militarily useful radiological weapons would use local (as opposed to world-wide) contamination, and high initial intensities for rapid effects. Prolonged contamination would also be undesirable. In this light Zn-64 is possibly best suited to military applications. As noted above ordinary "dirty" fusion-fission bombs have the highest initial radiation intensities and must also be considered radiological weapons.No cobalt or zinc bomb has ever been atmospherically tested, and as far as is publically known none have ever been built. In light of the ready availability of fission-fusion-fission bombs, it is unlikely any special-purpose fallout contamination weapon will ever be developed.